AIMS: Cerebral inflammation occurs in many neurological disorders, such as stroke and traumatic brain injuries. The first wave of inflammatory responses of glia appears very early but activation of glia is often highly restricted to the region of neuronal death . The progressive enlargement of infarct volume and the breakdown of neuron-glia interactions provide the opportunity for glia to be further activated, and this activation constitutes the second wave of cerebral inflammation. We hypothesize that neurons have the ability to reduce glial activation, and in turn to reduce the extent the second wave of cerebral inflammation. ACCOMPLISHMENTS: We report that the presence of neuronal cells reduced lipopolysaccharide (LPS)-stimulated production of nitrite and tumor necrosis factor- a (TNF-a) in mixed-glial cultures. Neuronal precursor cells from day 16 embryos were added onto a confluent layer of glia and were co-cultured for six days. LPS stimulated the production of nitrite, a stable metabolite of nitric oxide, and TNF-a by glia in a dose-dependent manner. The presence of neuronal cells shifted the dose-response curve to the right, suggesting that the responsiveness of glia to LPS was decreased. Both the magnitude and the time course of nitrite production were decreased in the co-cultures of neurons and glia. The decreased nitrite and TNF-a production were not due to the cytotoxicity of LPS as there was no significant increase of lactate dehydrogenase release into the medium. Immune suppression occurred in both microglia and astrocytes. Immunocytochemical analysis revealed that immunoreactivity for inducible nitric oxide synthase and F4/80, a mice monocytic antigen, was decreased in microglia. Furthermore, the addition of PC12 cells, a neuronal cell line, to the mixed glial cell cultures also significantly reduced nitrite production. These studies demonstrate that the presence of neuronal cells reduced glial inflammatory responses by decreasing the responsiveness of glia to the immune stimulant, LPS. As cerebral inflammation plays an important role in neurodegeneration in many neurological disorders, this study might provide insight for understanding the role of glia-neuron interactions in inflammatory responses in the brain.